Field of the Invention
The present invention relates to a stress monitoring device of elasto-magneto-electric (EME) effect type, and specifically to a non-destructive stress monitoring device for structural components of ferromagnetic materials.
Background of the Invention
Several stress monitoring instruments are presently available, such as the pressure sensor, the resistance strain sensor, the vibrating wire strain sensor, the optical fiber grating strain sensor, the piezoelectric acceleration sensor for measuring vibration frequency, and the Elasto-Magnetic cable force sensor based on elasto-magnetic effect. Measuring cable force by applying the pressure sensor is realized by using the precise pressure gauge or hydraulic sensor to measure the hydraulic pressure of the oil cylinder when the cable is stretched by the lifting jack. However, due to the characteristics of the pressure gauge, including the instability in the indicator reading, the necessity to convert the reading to the load value, and large influence by human factor, the pressure sensor is unavailable in the dynamic stress monitoring for the in-service structures. Measuring cable force by applying the resistance strain sensor is based on the principle that the wire resistance varies as the length of the wire changes. Measuring cable force by applying the vibrating wire strain sensor is based on the principle that the vibration frequency of the tensioned metallic string varies as the force resulted from the relative displacement of fixed ends of the string changes. Measuring cable force by applying the optical fiber grating strain sensor is based on the principle that the wavelength of light-wave passing through the optical grating varies with deformation of the optical fiber. In the above mentioned three methods for measuring cable force, the three strain sensors must be sticked on the surface of the structural component, or be welded to the surface of the structural component through the supporting device, or embedded into deforming body. Therefore, when using the above mentioned three methods, the installation is inconvenient and the measurement results are susceptible to influences of external factors. For in-service structures, the resistance strain sensor, vibrating wire strain sensor, or optical fiber grating strain sensor can only measure the changes of the strain/stress (namely, the increment) relative to initial strain/stress after installation or zero setting, but cannot measure the actual absolute value of the strain/stress. Monitoring cable force by measuring vibration frequency uses the quantitative relationship between the cable force and the vibration frequency to convert the vibration frequency tested by the acceleration sensor to the cable force. Because it is simple, cost-saving, and available in online monitoring in-service structures, the method of monitoring cable force by measuring vibration frequency is widely applied in practical engineering. However, there are disadvantages for the method of monitoring cable force by measuring vibration frequency: (1) the relationship between the cable force and the vibration frequency is affected by flexural rigidity, slope, sag, boundary conditions, and the vibration reducing and damping device of the cable, which causes errors in the conversion of the cable force; (2) the converted cable force is static value or averaged value, correspondingly, it is impossible to obtain the variations of the cable force in periodic vibration; and (3) the method is not suitable for monitoring stress of the structural components other than cable.
Measuring cable force by applying the Elasto-Magnetic force sensor based on the Elasto-Magnetic effect implements the principle that the magnetization characteristic changes when the ferromagnetic component placed in the magnetic field suffers the stress, and derives the cable force of calibrated ferromagnetic components. Since the method of using Elasto-Magnetic sensor to monitor cable force has advantages in monitoring the actual stress of the in-service structures and realizing non-destructive monitoring, the method overcomes the disadvantages of the other above mentioned methods and thus is a promising method in monitoring stress of the in-service steel structures. At present, there are mainly two types of such Elasto-Magnetic force sensor, namely, the sleeve-type sensor and the bypass-type sensor. On one hand, when used for monitoring stress of the in-service structures, the sleeve-type sensor needs in-situ winding coils, which leads to inconvenient time-consuming operation and heavy workload. Furthermore, because it is hard to control the quality of the coils of the sleeve-type sensor, the accuracy in measurement is low. On the other hand, the utilization of the bypass-type sensor is still in the stage of exploration and has not been promoted to engineering application because of the shortcomings due to the conduction yoke, including large size, heavy weight, and high production cost. No matter the sleeve-type or the bypass-type, the existent Elasto-Magnetic sensors use the secondary coil as the signal detecting element, which results in long measurement cycle (at least 10 seconds for each measurement). Therefore, the existent Elasto-Magnetic sensors cannot realize real-time monitoring and cannot monitor stress variations of the structure in the vibration process (under the action of seismic or/and strong winds). In addition, when using the existent Elasto-Magnetic sensors, it is demanded that the drive coil is large or the magnetic current is high in order to produce strong enough magnetic field, or that the turns of the secondary coil in a winding of a certain length are increased so that the secondary coil could generate sensitive-enough signals to increase the signal-to-noise ratio of the signals for monitoring stress. Furthermore, since the secondary coil is usually wound around the cylindrical support skeletons, the existent Elasto-Magnetic sensors merely detects magnetic field inside the coil, and thus the measured force is the average force of the structural component inside the coil. Hence, the current Elasto-Magnetic sensor could merely measure the uniaxial loads exerted on the components, mainly the cable force, which limits the application of the current Elasto-Magnetic sensor in the components of non-cylindrical cross-section or under complicated loadings.
As illustrated in FIG. 1, the conventional Elasto-Magnetic stress monitoring device generally comprises an excitation coil, a secondary coil, a support skeleton, a drive circuit, an integrator, a data acquisition and processing module, and a controlling instrument (such as a computer). The support skeleton is installed around the monitored structural component, and the excitation coil and the secondary coil are wound on the support skeleton. When the excitation coil is charged with electricity, a magnetic field is generated to magnetize the structural component to a nearly saturated state. Then, during the demagnetization phase, the magnetic flux passing through the secondary coil is changed and thus the secondary coil output an induced signal. And then, the induced signal is integrated to obtain a detectable electric signal. After data analysis and processing for the detectable electric signal, a characteristic value related to the permeability of the monitored structural component is achieved. Because the permeability of the monitored structural component is related to the stress state, the characteristic value can be converted to the stress using the beforehand calibrated data in the laboratory or on the site where the structural component is installed.
Several references in the art recite the Elasto-Magnetic stress monitoring device of FIG. 1, including CN 201242481Y, CN 101334325A, CN 101051226A, CN 101013056A, CN 24276011Y.